Natural killer cells (NK cells) provide an efficient effector mechanism by which immunosurveillance eliminates tumor or virally infected cells. A well-defined characteristic of NK cells is their ability to lyse target cells deficient in expression of MHC class I molecules. This observation has been basic for the identification of different inhibitory receptors expressed by NK cells. Upon binding to the MHC-class I molecules expressed on target cells, these receptors deliver inhibitory signals that down-regulate cytolytic functions. In humans, recognition of HLA-class I molecules is mediated by two types of receptors: those belonging to the Ig superfamily which include both KIR and LIR-1/ILT-2 proteins whose ligands are represented by various groups of HLA-A, -B and -C alleles, and the lectin-like CD94/NKG2A receptor complex which recognizes HLA-E molecules. The expression of these inhibitory receptors explains how NK cells can distinguish between HLA-deficient and normal cells. On the other hand, limited information existed in the activating NK receptors responsible for triggering the natural cytotoxicity. Only recently two distinct NK-specific receptors have been identified that play an important role in the NK cell mediated recognition and killing of HLA Class I defective target cells. These receptors, termed NKp46 and NKp44, are members of the Ig superfamily. Their cross-linking induced by specific mAbs leads to a strong NK cell activation resulting in increased intracellular Ca++ levels, in triggering of cytotoxicity and lymphokine release. Importantly, mAb-mediated masking of NKp46 and/or NKp44 resulted in inhibition of NK cytotoxicity against most, but not all, target cells. These findings, while providing evidence for a central role of NKp46 and NKp44 in natural cytotoxicity, also implied the existence of additional receptors.